DE 10 2010 023 571 A1 discloses a generic adjustable camshaft in the form of a built camshaft for an internal combustion engine, comprising an outer shaft and an inner shaft, which is arranged inside the outer shaft and is mounted such that it can rotate by a predefined angle in relation to said outer shaft, and at least two cams, a first cam being arranged in its entirety for conjoint rotation with the inner shaft and such that it can rotate relative to the outer shaft. The second cam has at least two part-cam elements such that a first part-cam element is arranged in a rotationally fixed manner on the outer shaft and a second part-cam element is connected in a rotationally fixed manner to the inner shaft and is arranged such that it can rotate relative to the outer shaft. This should allow the most variable control possible of the gas exchange valves of the internal combustion engine.
A further camshaft with a variable valve opening period is known from WO 2011/032632 A1.
It is often disadvantageous in the camshafts known from the prior art that they resort to two different cam profiles that make maximum opening of the valve possible. This produces a small peak, which then goes back down to a lower lift. However, this brings a comparatively large dynamic into the associated valve drive system and changes the valve lift during opening. For an Atkinson cycle process, for example, it is advantageous to leave the opening process unchanged and to hold the valve opening over a longer period.